Choose the appropriate number of nounse given in brackets. 1.These (phenomenon, phenomena) often occur in nature.2.Students in our group belong to various social(stratum, strata).3.They cannot discuss the matter until they get sufficient (data, datum).4.Politeness (formula, formulas) can be found in any book of etiquette.5.I feel that things are coming to a (crisis, crises).6.Success in money-making is not always a good (criterion, criteria) of real success in life.7.High salaries are an (indices, index) of the country's prosperity.8.The school (curricula, curriculum) is being discussed now.9.Mr. Clark is an expert in critical (analysis, analyses) of French literature.10.Your arguments do not have a firm (basis, bases).11.I'm afraid your (criteria, criterion) of morality are wrong.12.Professor Clare took part in three international (symposium, symposia) last year.